Electrographic recording process with charging deflection

ABSTRACT

THE IMAGE-WISE CHARGING TO RECORDING MATERIALS IN WHICH A CORONA DISCHARGE CURRENT CONSTANT IN TIME IS DIRECTED FROM AN ELECTRODE THROUGH A GAP IN A METAL SCREEN TOWARD A RECORDING MATERIAL. THE GAP HAS ALONG A LIP A MEANS FOR PRODUCING A VARIABLE ELECTRIC FIELD TRANSVERSELY ACROSS THE GAP WHICH VARIABLE ELECTRIC FIELD IS CAPABLE   OF DEFLECTING THE DISCHARGE CURRENT AND THEREBY CONTROLLING THE CHARGE IMAGE PRODUCTION.

July 20, 1971 FIG. 3

WALTER SIMM,

W. SIMM El"AL ELECTROGRAPHIC nmconnmei PROCESS WITH CHARGING DEFLECTIONFiled Nov. 13, 1968 4 16;!0 14 4 SL3 13 l5 l2- INVENTORJJ:

ROLF MULLER.

United States Patent US. Cl. 961 7 Claims ABSTRACT OF THE DISCLOSURE Theimage-wise charging of recording materials in which a corona dischargecurrent constant in time is directed from an electrode through a gap ina metal screen toward a recording material. The gap has along a lip ameans for producing a variable electric field transversely across thegap which variable electric field is capable of deflecting the dischargecurrent and thereby controlling the charge image production.

The invention relates to a process for the production of charge imageson insulating recording material, for example, on paper coated withsynthetic resin, in which the charging current required to form theimage-wise charging of the recording material is controlled by meansof-a control electrode.

Electrographic processes in which the charge images are produced oninsulating recording coatings using electrodes shaped according to theimage are already known.

In another known process the charge image is produced by an electrodewhich consists of numerous individual elements which are arranged in theform of a raster 'and which are insulated from each other, theseindividual electrodes being employed in a combination corresponding tothe print to be produced.

Processes in which image-wise charging of insulating recording materialsis produced using photoconductors in a discontinuous manner are alsoknown. In these processes, the charge is transferred from thephotoconductor to the recording material across an air gap which is situated between the photoconductor which serves as charging electrode andthe surface of the recording material. Jer-ky transmission of charges,which may cause lack of sharpness in the image, cannot be avoided underthese conditions.

The disadvantages of these processes are the considerable technicalcomplexity of a process which uses preformed electrodes and the factthat the other recording processes are not sufliciently reproducible,due to the fact that the initiation of each discharge depends to a largeextent on external conditions, for example the temperature and humidity.

The latter process also has the disadvantage that the jerky dischargescause the developed image to have a patchy structure, which impairs boththe power of resolution and the reproduction of half tones.

It is the object of the invention to develop a process in whichimagewise charging of the recording material can be influencedcontinuously by a control electrode so that the above disadvantages areavoided.

An electrographic process has now been found for the production ofcharge images on insulating recording materials by image'wise chargingof a neutral recording material or by imagewise discharge of ahomogeneously precharged recording material, using a corona dischargecurrent, which is characterised in that a corona discharge currentemitted from a corona discharge which is kept constant in time, whichdischarge is produced from a discharge 3,594,162 Patented July 20, 19,71

electrode, is used, which discharge current is directed through the gapof a metal screen onto the surface of the recording material whilst atransverse imagewise differentiated electric field is produced on acontrol element which is arranged on one of the lips of the gap.

According to one embodiment of the invention, the recording material ischarged imagewise by a constantly maintained corona discharge current.In this process, part of the corona discharge current emitted from adischarging electrode is directed onto the surface of the recordingmaterial through a gap in a metal screen whilst a transverse electricfield is produced in the gap by the electrical charging of aphotoconductor strip arranged on one of the lips of the gap. Thistransverse field is differentiated by the imagewise exposure to light ofthe photoconductor strip, i.e. it is rendered permeable or impermeableto the corona discharge current, which amounts to an image wise controlof the discharge current. In addition, the potential of thephotoconductor can be adjusted to the best value for image recording bymeans of an auxiliary electrode embedded in the photoconductor strip.

According to a further embodiment of the invention, the photoconductorstrip which acts as the control element is replaced by a flat bundle oflinear conductive elements over which the transverse field can becontrolled by electrical signals, for example by video signals or byvoltage impulses which are used for transmitting measuring values.

The invention is described in more detail below with reference to thedrawings. In particular, FIG. 1 illustrates the principle of the processaccording to the invention on a simple embodiment given as an example,

FIG. 2 shows another embodiment of the process, which uses a slidingexposure of the photoconductor strip,

FIG. 3 shows a useful design for the discharge electrode 1,

FIGS. 4 and 5 are two cross-sections of embodiments of the controlelectrode given as examples for the production of the lip of a gapprovided with a photoconductor,

FIG. 6 shows a control electrode which comprises numerous conductorelements instead of the photoconductor strip.

FIG. 1 illustrates the principle of the process according to theinvention. A corona discharge is produced on the discharge electrode 1,which may be in the form of a thin Wire or a series of metal tips, byapplying a voltage from the high voltage source 2. From this discharge,a current passes over from the charge carriers to the metal screen 4,and part of the current passes through the gap 5 to the recordingmaterial 6 where a linear or band shaped charged strip 7 is produced,which depends on the form of the gap. The recording material, e.g. apaper coated with synthetic resin, is supported by the grounded metalplate '8. To facilitate the passage of the charge through the gap 5, avoltage from the voltage source 9 is applied to the screen 4 and theplate 8. The size of this voltage is calculated to provide optimumcharging of the recording material 6 but without sparking over ortransmission of charges from the screen to the surface of the recordingmaterial.

One lip of the gap 5 is covered with a strip of photoconductor 10. Owingto the relatively high dark resistance of the photoconductor, apotential is built up on the unexposed photoconductor by the chargecarriers which strike it, and this potential produces an electric fieldtransversely across the gap, as indicated by the lines of force 11.Where such a transverse field exists, the charge carriers meeting thegap are deflected sideways and taken up by the screen. Charging of therecording material 6 is thereby prevented. When the photoconductor isexposed, its conductivity is sufiicient to deflect the charge carrierssprayed on it to the screen, and no transverse field is produced in thegap 5. Whether the recording material 6 is charged or not thereforedepends on the exposure of the photoconductor 10 to light. Now only apart or several parts of the strip 10 can be reached by the exposure,with the result that linear charging on the recording material 6 is onlypartial, e.g. corresponding to a narrow zone passing transverselythrough an optical image which is exposed to direct light. The imageline is then reproduced on the recording material as the line of acharge image.

As shown in FIG. 2, a stationary image projected into the plane of thegap can be scanned by the gap of the screen 4 over the entirecross-section of the material 6 by shifting the screen 4 in one of thedirections indicated by the arrows, and this image can then betransmitted as a charge image to the recording material 6. This chargeimage is then rendered visible in known manner by toning. Scanning ofthe optical image can also be carried out by holding the screen 4 inposition but synchronously moving the projected image and the recordingmaterial 6.

To improve the spraying of the discharge electrode 1, it is advantageousto surround the electrode with a metal jacket 12 (FIG. 3) mounted on thescreen 4 over the gap 5. The metal jacket is open at the top and bottomso that the gap 5 can be exposed to light from above.

The higher the resolution required of the developed film, the lower thephotoconductor strip should be so that conductive bridges from the edgeof the photoconductor strip to the connection to the metal screen can beestablished even by small areas of the projected image and the chargescan be removed at these points. However, the narrower the strip 10, thesmaller will also be the charge on the surface of the strip in theunexposed state and 'hence also the controlling power of thephotoconductor.

Extremely narrow photoconductor strips can, however, be used if thephotoconductor strip is equipped with an additional electrode to whichan auxiliary-voltage can be applied by means of which the controlpotential can be adjusted to the desired value. Suitable embodiments ofthe strips are shown in FIGS. 4 and 5.

In FIG. 4, the part of the screen which carries the photoconductor is inthe form of a rail 12 made of synthetic resin which forms a step at 13.The recessed part if filled with the photoconductor 10 in the form of abeam of square cross-section. The top of the beam is connected to themetal body 4' of the screen by means of a conductive cover 14 which hasbeen painted to just before the edge of the beam 1-0 so that only a verynarrow strip remains uncovered. The charge is carried away over thiscovering when-the photoconductor is in the exposed condition. Supply ofthe charge and maintaining the potential in the dark are eifected by theconductive wire 15 embedded in the photoconductor. The edge prepared inthis way forms a lip of the gap 5. The second lip 16 of the gap is madeof metal and is arranged opposite the free edge of the photoconductor10.

FIG.. 5 shows another embodiment of the control electrodes, in which therecess of the synthetic resin rail 12 is filled with photoconductor 10in such a way that the cross-sectional surface is a triangle one edge ofwhich forms the gap together with the opposite metal strip 16.

Control electrodes of the type illustrated in FIGS. 4 and 5 which areequipped with auxiliary electrodes no longer need the corona dischargecurrent for charging them since the transverse field can be adjusted inthe gap at will by means of the auxiliary electrode by applying asuitable auxiliary voltage. In this form, the control electrode is alsosuitable for operation with alternating current. If the auxiliaryvoltage used in an alternating voltage of sufficiently low frequency, eg50* cycles per second, charge structures in the form of a line rasterare produced on a carrier which is moved relatively to the gap, andthese structures may be advantageous for XC1uSiV6 velopment.

Control of the charge current can also be achieved if, instead ofpreparing the photoconductor on a lip of the gap, linear conductors 17are arranged on the gap 5 of the screen 4 in FIG. 6-, across whichvoltage impulses or control voltages can be applied for image recording.This control electrode may be, for example, in the form of a flat bundleof wires or metal coatings on synthetic resin similar to printedcircuit. Connection to a scanning de vice can be established overcomparatively large distances through the conductor elements, forexample to allow the transmission and recording of images or measuringvalues.

In the apparatus described in FIGS. 1 to 6, the width of the gap is afew tenths of a millimetre, preferably 0.2 to 0.5 mm. The distance ofthe screen 4 from the surface of the recording material 6 amounts to afew millimetres, preferably 1 to 2 mm. The control voltages at the edgesof the gap is in the region of 0 to 1000 v., preferably 0 to 300 v. Thevoltage between screen 4 and the support 8 is adjusted to the optimumvalue such that interfering transmission of charge from the screen tothe surface of the carrier does not occur. It is in the region of a fewk-volts. The corona discharge on the discharge electrode 1 is producedin a known manner, the voltage being adjusted to the optimum valueaccording to the geometrical relationships of the arrangement.

The process according to the invention can, of course, also be used forimagewise discharging of a homogeneously precharged recording material,the corona discharge current causing eithcr imagewise neutralisation ofthe charges already present or an additional imagewise charging of therecording material with a charge of opposite sign.

The process according to the invention has considerable advantages overthe conventional processes, consisting inter alia in that images can berecorded with simple electrode arrangements and the desired effect isachieved with minimum expenditure in apparatus. Furthermore, paper whichdoes not have a perfectly flat surface can be used as the recordingmaterial since the quality of the recording is substantially unaffectedby the distance of the control electrodes from the surface of therecording material. This opens up the possibility of obtainingsuperimposed development of several separated images in differentcolours since one insulating toner which has been applied to the paperdoes not impair a second imagewise charging. The distance of theelectrodes can always be kept sufliciently large that any toner imagealready present will not be wiped out. Other advantages lie in theoperational reliability of the process and the possibility of thecontinuous application of image charges.

What is claimed is:

1. An electrographic process for the production of charge images oninsulating recording material by imagewise charging of a neutralrecording material, or by imagewise discharging of a homogeneouslyprecharged recording material, using a corona discharge current,comprising the steps of producing a corona discharge from a dischargeelectrode, said corona discharge being constant in time, directing thecorona discharge to a surface of a recording material positioned toreceive said discharge current, passing said corona discharge current tosaid surface through a metal screen interpositioned between saiddischarge electrode and said recording material surface passing thecurrent through an elongated gap defined in the screen, producing anelectric field transversely across the gap by a lamellar electrode atone longitudinal side of the gap deflecting the discharge current bysaid electric field provided from said lamellar electrode by varyingpotentials in said strip and producing a control of the charge imageproduction on the recording material surface.

2. A process according to claim 1 wherein said transverse field issupplied by applying a potential to a conductive strip at onelongitudinal side of said gap.

3. A process as claimed in claim 1 in which said transverse field isproduced across said gap by applying a potential to several linearconductors terminating at said gap.

4. The process of claim 1 wherein said transverse field is supplied byapplying an insulating photoconductive material on one longitudinal sideof said gap, said photoconductive material holding the charge on itsinsulating surface thereby establishing said field, and wherein thephotoconductive material is exposed to a light image to remove thetransverse field allowing charges to be deposited on the recordingmaterial surface.

5. The process according to claim 4 wherein potential is applied to aconductor embedded in the photoconductive material to maintain thetransverse field.

6. A process according to claim 1, characterised in that a voltage whichis lower than the breakdown voltage of the air layer between screen andrecording material is applied between the metal screen and theconductive support of the recording material.

7. A process according to claim 1, characterised in that the recordingmaterial on the one hand and the screen and the discharge electrode onthe other hand are moved relatively to each other during the recordingprocess.

References Cited UNITED STATES PATENTS GEORGE F. LESMES, PrimaryExaminer J. C. COOPER III, Assistant Examiner U.S. Cl. X.R. 250-495;346-1, 74; 355-17

